People use the word whale in different ways. Some use it for all Cetaceans including dolphins and porpoises. These people say that dolphins and porpoises were also whales, because they are also Cetaceans. Others separate out the dolphins and porpoises: common English-speaking people have never called them or thought of them as whales unless they are very large. Actually, there is no clear line between whales and dolphins.

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Baleen whales eat plankton: clouds of very small things floating in the water, and krill. The whales' mouths are very large. They open their mouths very wide and hold a big mouthful of sea water. Their throats stretch very wide to make the space inside their mouth even bigger. Then they close their mouth and squeeze out the seawater. The food does not escape because, instead of teeth, these whales have filters called baleen. Baleen is long, hard strips that act like a strainer. The water goes through the baleen. Animals and plants in the water are trapped and swallowed, while the water goes back out.[2] This is very different from the way that toothed whales eat.

Toothed whales eat larger fish or meat and are like big dolphins. They have sharp teeth and usually have a big forehead. Inside the big forehead is a chamber to make and direct sounds. They make all kinds of sounds, including sounds so loud they can shock fish. They can use echolocation to locate things that they can not see. Some toothed whales, such as the sperm whale, are almost never called dolphins. Some of them are always called dolphins. Others are like dolphins in some ways and like whales in others.

All cetaceans, including whales, dolphins and porpoises, are descendants of land-living mammals of the Artiodactylorder (even-toed ungulates). Both are related to Indohyus (an extinct semi-aquatic deer-like ungulate) from which they split around 54 million years ago.[4][5] Primitive whales probably first took to the sea about 50 million years ago and became fully aquatic about 5-10 million years later.[6]Pakicetus is an important transitional species.

Because of where they live (and unlike many animals), whales are conscious breathers: they decide when to breathe. Whales breathe through blowholes. Baleen whales have two and toothed whales have one. These are on the top of the head: the animal breathes while most of their body is underwater. Breathing first shoots out extra water from the blowhole, making a jet into the air, followed by inhaling air into the lungs.

All mammals sleep, including whales, but they cannot stay in an unconscious state for too long, because they need to be conscious to breathe. It is thought that only one hemisphere (half) of their brains sleeps at a time, so that whales are never completely asleep, but still get the rest they need.[7] Whales are thought to sleep around eight hours a day.

A single calf is born every two or three years. Gestation takes up to a year. Nursing continues for more than a year in many species; there is a strong bond between mother and calf. Reproductive maturity occurs at seven to ten years. This mode of reproduction produces few offspring, but increases their chance of surviving. The effect of hunting is severe on these animals, who replace their numbers so slowly.[8]

Breaching is what happens when a whale jumps into the air and then purposefully flops down on the water with a great splash. Sometimes it twirls in the air when it does this. Scientists are not sure whether breaching is done to play, to clean the whale's skin of things that are stuck to it, or to tell other whales something.[10]

There are a number of other behaviors on the surface which are not well understood.[11] 'Logging' is when a whale swims slowly at the surface of the ocean with very little movement. When a whale does this, it looks like a log in the water. Some scientists think this is a kind of rest or sleep for whales. 'Spyhopping' is when a whale sits straight up in the water with its head straight up and out of the water. It will sometimes turn around in circles as it spyhops. Some scientists think this might be because whales are trying to see what is happening above water. 'Lobtailing' is done when a whale faces downward in the water. It then slaps the water with a thunderous sound. Scientists think this might be done to warn other whales of danger, or as a method of feeding.[12]